In the manufacture of semiconductor devices, there is a need to make electrical contact to certain regions of the device. Normally, it is necessary to make contact to device regions underlying a dielectric on the surface of the silicon substrate by first forming an opening or via in the dielectric over the region to be contacted and next depositing a conductive material over the substrate surface and within the opening or via. The conductive material is then patterned for connecting different parts of the integrated circuit (that is, to form “interconnects” or “interconnect lines”). Traditionally, sputtered aluminum (Al) has been used as the conductive material. Typically, a blanket layer of aluminum is deposited on the surface of the silicon substrate covering any overlying dielectric as well as device regions exposed by openings in the dielectric. This is followed by a masking step that leaves photoresist positioned covering the openings or vias to the regions to which interconnects are formed. An etch step removes the aluminum from the areas not covered by photoresist, wherein the aluminum that is not removed by the etch step fills the openings or vias and forms the interconnect lines, thereby making electrical contact to the desired region.
As geometries have decreased to submicron levels and devices have become more densely packed on the substrate surface, the openings or vias to the device regions to be contacted have increasingly greater aspect ratios (ratio of height to width). Aluminum deposition alone has proven to be inadequate in devices with high aspect ratios. The problems encountered include poor step coverage, poor contact integrity, and inadequate planarity.
To overcome these shortcomings, tungsten and other refractory metals are used as a contact for devices with submicron contacts before aluminum deposition and patterning.